The present invention relates to a color printer, color copier or similar electrophotographic color image forming apparatus and more particularly to a color image forming apparatus capable of sensing the misregister of colors and correcting it.
A color image forming apparatus of the kind described includes a plurality electrophotographic image forming sections arranged along a conveyor belt. Images formed by the image forming sections are sequentially transferred to a paper or similar recording medium being conveyed by the conveyor belt one above the other, completing a color image on the paper. A problem with an electrophotographic image forming apparatus is that the dimensional errors of mechanisms and the errors of a driveline cause a toner image to be formed at a position different from an expected position, resulting in irregular images. Particularly, in a color image forming apparatus sequentially transferring images of different colors to a single recording medium one above the other, the relative positional deviation of the images of different colors occurs. The deviation directly translates into a conspicuous color deviation and degrades image quality to a critical degree. This is particularly true with a full-color image forming apparatus including a plurality of photoconductive elements.
More specifically, in the above color image forming apparatus, the distance between the axes of nearby photoconductive elements is not constant throughout the image forming sections. Further, the degree of parallelism differs from one photoconductive element to another photoconductive element. Moreover, exposing devices involve various errors including the positional error of optics including a mirror and an error in write timing. These errors cause the toner images of different colors, or toner, to be deviated from each other on a paper. Although such misregister may be corrected by initial adjustment, it again occurs later due to the displacement of an image forming unit accommodating the image forming sections, maintenance, transport and so forth. In addition, the errors vary with the elapse of time due to the thermal expansion of mechanisms ascribable to temperature elevation that is caused by repeated image formation. It follows that the apparatus must be adjusted at short intervals.
To solve the above problem, Japanese Patent No. 2,633,877, for example, discloses a color misregister sensing and correcting method using register marks, or toner marks, and a mark sensor. The mark sensor, implemented by a CCD image sensor by way of example, senses the register marks formed on a conveyor belt so as to recognize the positions of images of different colors. However, this method is not practicable without allocating a substantial period of time for the operation for sensing and correcting a color deviation and is therefore likely to lower image forming efficiency, as will be described specifically later.
In light of the above, various implementations for promoting efficient register of images of different colors have been proposed in relation to a color image forming apparatus. For example, Japanese Patent Laid-Open Publication No. 6-51607 proposes to execute frequent correction when the deviation of register noticeably varies or increase the intervals between correction when the deviation is small. Japanese Patent Laid-Open Publication No. 7-234612 proposes to form, when the number of prints exceeds a preselected number in a repeat print mode or when the output of an environment sensor exceeds a preselected value, a pattern image between consecutive papers so as to sense and correct misregister. Japanese Patent Laid-Open Publication No. 8-272936 proposes to sense temperature in an exposing section and temperature around an apparatus and control a correcting operation on the basis of a different between them. Further, Japanese Patent Laid-Open Publication No. 9-244332 proposes to control a correction timing by selectively using sequentially varying preselected intervals and a constant interval.
The above conventional schemes are directed toward an optimal register timing that implements efficient register. Even such schemes, however, bring about the scatter of the results of sensing and cannot easily estimate a timing for correction on the basis of the previous correction.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 10-213943.
It is therefore an object of the present invention to provide a color image forming apparatus capable of efficiently sensing a color deviation and accurately correcting it. A color image forming apparatus of the present invention includes a plurality of electrophotographic image forming sections arranged along a conveyor belt for forming images, and sequentially transfers the images to a single recording medium being conveyed by the conveyor belt one above the other to thereby form a color image on the medium. A register mark forming device causes each of the image forming sections to form a respective register mark on the conveyor belt. A mark sensor adjoins the conveyor belt for sensing the register mark. A correcting device corrects a deviation between colors in accordance with the result of sensing of the register mark output from the mark sensor. A storage stores past deviations between colors sensed by the mark sensor. The correcting device executes the correction of a deviation between colors on the basis of the past deviations stored in the storage.